1. Field of the Invention
The present invention is directed to an apparatus for forming and retaining a core sample within a downhole tool where said apparatus is adapted to be coupled to a conventional coring assembly. More specifically, the present invention is directed to a full closure valve and core retention assembly for use in forming and collecting unconsolidated or loosely consolidated core samples where said valve is externally actuated by surface pump pressure.
2. Background
To analyze the amount of hydrocarbons which are contained in a soil at a particular depth in the proximity of a subterranean well generally requires the extraction of a sample of well material. An analysis of this material yields the percent of fluid and/or gas contained in the sample, which information may then be utilized to determine the type of fluid contained therein. A core analysis is also valuable from the standpoint of the stratigraphic analysis of the core itself. In view of the cost of extracting the core, which may be considerable in the case of a deep well, it is generally important to extract the largest core sample possible. It is also important to retrieve the sample without substantial stratigraphic disturbance.
Coring is a common practice in the field of petroleum exploration. However, it is not uncommon to encounter formations which are considered impossible to core because of their unconsolidated nature. For example, oil-sand, water-sand, or loose debris constitute types of unconsolidated formations which are commonly found in the field and which are extremely difficult to core. Even if the loose, unconsolidated material can be successfully cored or cut, the problem still remains as to how to remove such material from the bore hole in a manner such that its original stratigraphic orientation is undisturbed. Typically in prior art core catcher assemblies, unconsolidated material is prone to drop out of the core barrel when the core barrel is lifted to the surface of the well.
A number of core catchers have been designed which utilize a collection of arcuate wedge shaped segments in a "clam shell" or "flapper valve" type of arrangement. In such designs, the valve segments are designed to move apart so as to allow the upward movement of the core within the core barrel. When the core undergoes downward movement, however, the valve segments engage the core and rotate to a shut or fully closed position. This downward movement of the valve segments is typically gravitationally induced.
Disadvantages of such clam shell or flapper valve designs involve their propensity to create an obstruction upon which the core may jam as it is being cut. Obstruction or disturbance of the core is further increased by the use of spring biased flapper valves which are sometimes used to increase the reliability of the system. Alternatively, valve segments of such designs are often prone to disturb the original stratification of the core sample.
Further disadvantages of prior art closure valves involved their manner of actuation. Prior art closure valves typically involve a closure mechanism which is gravitationally induced, as above described, or which is actuated by variations in fluid flow to the tool. An example of a fluid actuated closure valve is seen in U.S. Pat. No. 4,522,229. Fluid actuated closure valves such as those disclosed in the '229 patent are generally biased in an open position by fluid flow through the bore of the tool. When a core sample has been taken, the valve is closed by channeling or rerouting fluid flow so as to induce valve closure. This is typically accomplished by using a drop ball sub at the surface and physically dropping a check ball through the bore defined in the workstring. This ball descends through the well fluid in the bore of the tool and ultimately comes to rest in the main fluid portal so as to restrict fluid flow therethrough. This channeling of fluid flow results in the closure of the valve.
Disadvantages with this type of system include the inconvenience and time associated with breaking the workstring at the surface. Due to reliance on gravity, this type of technique, as well as flapper or clam shell designs, are also largely ineffectual in horizontal wells.